Assorted fixes

• Remove unnecessary casts from malloc() & memcpy() calls
• rgb.c: fix -Wdiscarded-qualifiers warning
• rgb.c: if filename is too long, report error instead of truncating
• README: provide more information on --with-rgb-db-type
• configure: Add --with-rgb-db-library option
• gitlab CI: build all 3 --with-rgb-db-type=(text|dbm|ndbm) options